[harpastum]

Well, I think the reason most people try to be nice/humorous is that the screen-poker is generally not aware that they are doing something wrong. They view their computer as a piece of furniture--something that sits on their desks.

Making a joke or lighthearted comment is a lot more effective than saying "hey there, I know you just use your work computer for MS Office, solitaire, and Facebook, but I care more about this computer than most people care for their children. Keep your dirty mitts off of it."

They'll probably never understand why you care so much for your computer, so it's better just to pretend you have some 'weird' issue with people touching your stuff.

[skwaddar]

If they are not an asshole "please don't touch my screen" is all you need to say.